This investigation is concerned with antigenic properties of a macromolecular substance located on the surface or inside an intact cell. Regulation of the immune response of this type of antigenic mosaic is very complex and involves numerous factors. In our studies we are concerned with cellular interactions which seem to be essential in the immunoregulation. Bacterial cell provided us with a useful model since it possesses a variety of antigens both thymus-dependent and independent. We are also concerned with the class of antibody which is formed as a result of the cellular stimulation and the effect it will have on formation of other classes of antibody. Since we were successful to develop techniques to culture lymphocytes in vitro which produce antibody to microbial antigens a detailed analysis of factors, both cellular and humoral, that regulate these immune responses is actively being investigated.